The invention relates to a low-pressure mercury vapour discharge lamp having an envelope which is sealed in a vacuum-tight manner and transmits radiation, and which contains mercury and a rare gas, the envelope being provided on its inside with a luminescent layer comprising a green luminescing zinc orthosilicate activated with bivalent manganese, defined by the formula Zn.sub.2 SiO.sub.4 :Mn.sup.2+.
A low-pressure mercury vapour discharge lamp of the type described above is known from the book by Keith H. Butler, "Fluorescent Lamp Phosphors", 1980, pp. 6 and 8.
Excitation of the manganese-activated zinc orthosilicate (willemite) with short-wave ultraviolet radiation from the mercury discharge, mainly 254 nm radiation, generates an emission in the green part of the spectrum.
Lamps emitting such a green light are used in practice mainly for achieving colour effects, for example in stage lighting, shop window lighting, festive illumination, etc.
The envelope, which is usually made of glass, may be designed in various shapes, for example as a straight tube, a U-shaped tube, or a torus-shaped tube. The envelope may also be designed as a bulb having an indentation as with an electrodeless lamp. Before the luminescent layer is applied, the envelope may have been coated over part of its interior circumference with a reflecting layer, so that the green radiation generated emanates through a window left open by the reflecting layer. If so desired, it is also possible to provide the tube on the inside with an absorptive layer which absorbs undesirable mercury lines in the blue and/or long-wave ultraviolet part of the spectrum.
A disadvantage of the known lamp is that during operation of the lamp the luminescent layer shows a strong greying. The greying effect during operation causes the lamp to look black when not operating, which is visually unattractive and in some lamp applications undesirable. Moreover, the greying effect causes a shift of the colour point of the emitted radiation and a reduction of the light output. It is assumed that the greying effect is caused by zinc ions of the luminescent willemite being reduced to metallic zinc during operating of the lamp. The metallic zinc can subsequently form an amalgam with the mercury from the discharge.